The present invention relates to a nematic liquid crystal composition. More specifically, the present invention relates to a liquid crystal composition for an active matrix liquid crystal display (AM-LCD) and a liquid crystal display using this liquid crystal composition.
At present, a transmissive AM-LCD using a back light has been put to practical use as a matrix liquid crystal display capable of displaying full color in various fields such as computer terminals, car navigation systems and the like. However, it has the defect that this back light consumes a large electricity, and brought about is the inconvenience that digital still cameras and digital video cameras which are loaded with the transmissive AM-LCD can not be used for long time.
In order to solve this problem, a reflective AM-LCD using no back light has been developed. As reported in SID97 Digest, p. 643 by S. -T. Wu, C. -S. Wu and C. -L. Kuo, light passes twice through a liquid crystal layer in the reflective AM-LCD, and therefore a product (xcex94nxc2x7d) of a thickness (d) of the liquid crystal layer and a birefringence (xcex94n) of the liquid crystal has to be set to a lower level. To be specific, a xcex94n required to a liquid crystal used for an AM-LCD of a conventional transmissive TN type has been roughly 0.075 to 0.120 but in the case of an AM-LCD of a reflective TN type, a xcex94n required to a liquid crystal is 0.075 or less.
Characteristics other than a xcex94n required to a liquid crystal composition for a reflective AM-LCD include the following items (1) to (4):
(1) In order to improve a contrast of the LCD, the composition has a high resistivity and a high voltage-holding ratio (VHR).
(2) In order to make it possible to use the LCD outdoors, the temperature range showing a nematic phase is wide (the upper limit temperature showing a nematic phase is high, and the lower limit temperature showing a nematic phase is low).
(3) In order to reduce an electricity consumed by the LCD, the threshold voltage (Vth) is low.
(4) In order to shorten the response time, the viscosity (xcex7) is low.
Literatures disclosing liquid crystalline compounds or liquid crystal compositions which are considered to be usable for an AM-LCD include, for example, Japanese Patent Application Laid-Open No. 29771/1999, Japanese Patent Application Laid-Open No. 245559/1998, Japanese Patent Application Laid-Open No. 255956/1997 and Japanese Patent Application Laid-Open No. 249881/1997. However, these disclosed liquid crystal compositions have the defects that the xcex94n is large or the xcex94n is relatively small but the lower limit temperature showing a nematic phase is high and the voltage-holding ratio is low, and therefore it has been unsatisfactory for use in an AM-LCD of a reflective TN type.
As described above, the liquid crystal compositions for an AM-LCD have intensively been investigated according to various purposes, but the existing state is that they are always. requested to be newly improved.
An object of the present invention is to provide a liquid crystal composition which has particularly a high upper limit temperature of a nematic phase, a low lower limit temperature of the nematic phase and a small birefringence while satisfying general characteristics required to the liquid crystal composition for an AM-LCD described above.
Intensive investigations repeated by the present inventors in order to solve these problems have resulted in finding that the expected object can be achieved by a liquid crystal composition comprising a component I comprising at least one compound selected from the group of compounds represented by Formula (I-1) or (I-2), a component II comprising at least one compound selected from the group of compounds represented by Formulas (II-1) to (II-8) and a component III comprising at least one compound selected from the group of compounds represented by Formulas (III-1) to (III-5). Thus, they have come to complete the present invention.
The liquid crystal composition of the present invention is shown by the following items (1), (2) and (3):
(1) A liquid crystal composition comprising a component I comprising at least one compound selected from the group of compounds represented by Formula (I-1) or (I-2), a component II comprising at least one compound selected from the group of compounds represented by Formulas (II-1) to (II-8) and a component III comprising at least one compound selected from the group of compounds represented by Formulas (III-1) to (III-5): 
xe2x80x83wherein R1 and R2 each represent independently an alkyl group, an alkoxy group, an alkoxymethyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R3 represents an alkyl group, an alkoxy group, an alkoxymethyl group having 1 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms or xe2x80x94COOxe2x80x94R6; R6 represents an alkyl group having 1 to 10 carbon atoms; R4 represents an alkyl group, an alkoxy group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; R5 represents an alkyl group, an alkoxy group, an alkoxymethyl group having 1 to 10 carbon atoms or an alkenyl group having 2 to 10 carbon atoms; Z1 and Z4 each represent independently a single bond or xe2x80x94CH2CH2xe2x80x94; Z2 and Z3 each represent independently a single bond, xe2x80x94CH2CH2xe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94; X1 represents F, Cl, OCHF2 or OCF3; X2 represents F, OCHF2 or OCF3; Y1 and Y2 each represent independently H or F; and Q represents H or F.
(2) The liquid crystal composition as described in the above item (1), wherein the component I has a content of 3 to 30% by weight; the component II has a content of 5 to 80% by weight; and the component III has a content of 3 to 70% by weight each based on the whole weight of the liquid crystal composition.
(3) The liquid crystal composition as described in the above item (1) or (2), wherein a nematic phase has an upper limit temperature of 70xc2x0 C. or higher, a lower limit temperature of xe2x88x9220xc2x0 C. or lower and a birefringence of 0.075 or less.
The liquid crystal display of the present invention is shown by the following item (4):
(4) A liquid crystal display comprising the liquid crystal composition as described in any of the above items (1) to (3).
The component I in the liquid crystal composition of the present invention comprises at least one compound selected from the group of the compounds represented by Formula (I-1) or (I-2). The compound represented by Formula (I-1) has a birefringence (xcex94n) of almost 0, a clearing point (Tc) falling in a range of 10 to 30xc2x0 C. and a dielectric anisotropy (xcex94∈) of almost 0 and is excellent in a heat stability, a chemical stability and a compatibility. The compound represented by Formula (I-2) has a xcex94n falling in a range of about 0.06 to 0.07, a Tc falling in a range of 140 to 160xc2x0 C. and a xcex94∈ of almost 0 and is excellent in a heat stability, a chemical stability and a compatibility. This allows the component I to assume a role to reduce a xcex94n while maintaining particularly a high Tc in a liquid crystal composition for a TFT to which a high reliability is required. However, a composition prepared only from these compounds not only has a too narrow temperature range showing a nematic phase of the composition but also is increased too much in a threshold voltage and therefore is not preferred.
The component II in the liquid crystal composition of the present invention comprises at least one compound selected from the group of the compounds represented by Formulas (II-1) to (II-8). The compounds represented by Formulas (II-1) to (II-8) have a Tc falling in a range of roughly xe2x88x9250 to 160xc2x0 C., a xcex94∈ falling in a range of roughly 5 to 20 and a xcex94n falling in a range of roughly 0.03 to 0.12 and is excellent in a heat stability, a chemical stability and a compatibility. This allows the component II to assume a role to reduce particularly a threshold voltage in a liquid crystal composition for a TFT to which a high reliability is required. However, a composition prepared only from these compounds not only is deteriorated in a compatibility of the composition but also is increased in a xcex94n and a viscosity and therefore is not preferred.
The preferred compounds of the component II include the following compounds. R1 in the formulas is synonymous with the definition described above. 
The component III in the liquid crystal composition of the present invention comprises at least one compound selected from the group of the compounds represented by Formulas (III-1) to (III-5). The compounds represented by Formulas (III-1) and (III-2) have a Tc falling in a range of roughly 0 to 60xc2x0 C., a xcex94∈ of almost 0 and a xcex94n falling in a range of roughly 0.03 to 0.07, and it has particularly a low viscosity and is excellent in a heat stability, a chemical stability and a compatibility. The compounds represented by Formulas (III-3) to (III-5) have a Tc falling in a range of roughly 140 to 260xc2x0 C., a xcex94∈ of almost 0 and a xcex94n falling in a range of roughly 0.12 to 0.16 and is excellent in a heat stability, a chemical stability and a compatibility. This allows the component III to assume a role not only to elevate a Tc of the composition but also reduce a viscosity thereof. However, a composition prepared only from these compounds is increased too much in a threshold voltage in a certain case and therefore is not preferred.
The preferred compounds of the component III include the following compounds. R1 and R3 to R6 in the formulas are synonymous with the definitions described above. 
In the present invention, the liquid crystal composition for an AM-LCD having a small xcex94n, a small viscosity and a broad nematic phase range can be prepared by optionally combining the component I, the component II and the component III. That is, capable of being obtained is the liquid crystal composition which has a Tc falling in a range of roughly 70 to 100xc2x0 C., a xcex94n falling in a range of roughly 0.05 to 0.75 and a threshold voltage falling in a range of roughly 1.0 to 2.5 V and which has a low viscosity, a broad nematic phase range and a high voltage-holding ratio.
The contents of the respective components in the liquid crystal composition of the present invention shall be explained. The component I has preferably a content of 3 to 30% by weight, more preferably 5 to 28% by weight based on the whole weight of the liquid crystal composition. If it is less than 3% by weight, the liquid crystal composition is increased in a xcex94n in a certain case, and therefore it is not preferred. On the other hand, if it exceeds 30% by weight, the liquid crystal composition is degraded in a compatibility at a low temperature in a certain case, and therefore it is not preferred as well.
The component II has preferably a content of 5 to 80% by weight, more preferably 10 to 80% by weight based on the whole weight of the liquid crystal composition. If it is less than 5% by weight, the liquid crystal composition is increased too much in a threshold voltage, and therefore it is not preferred. On the other hand, if it exceeds 80% by weight, the liquid crystal composition is increased in a xcex94n as well as a viscosity in a certain case, and therefore it is not preferred as well.
The component III has preferably a content of 3 to 70% by weight, more preferably 5 to 70% by weight based on the whole weight of the liquid crystal composition. If it is less than 3% by weight, the liquid crystal composition is degraded in a compatibility at a low temperature in a certain case, and therefore it is not preferred. On the other hand, if it exceeds 70% by weight, the liquid crystal composition is increased in a threshold voltage in a certain case, and therefore it is not preferred as well.
Next, the characteristic values of the liquid crystal composition constituting the present invention shall be explained in detail.
A display using a liquid crystal composition in which an upper limit temperature of a nematic phase is lower than 70xc2x0 C. and a lower limit temperature of a nematic phase is higher than xe2x88x9220xc2x0 C. is limited in an environmental temperature. In particular, if it is used outdoors, there is the possibility that it can not be displayed and the function as a display can not be carried out. Accordingly, the nematic phase range of the liquid crystal composition is preferably set to 70xc2x0 C. or higher for an upper limit temperature showing a nematic phase and xe2x88x9220xc2x0 C. or lower for a lower limit temperature.
When using the liquid crystal composition in which a xcex94n determined on the conditions of 25xc2x0 C. and xcex=589 nm is larger than 0.075, a white display in a reflective AM-LCD is tinged with yellow in a certain case. Accordingly, the liquid crystal composition has preferably a birefringence of 0.075 or less.
The compounds constituting the components of the liquid crystal composition of the present invention can be synthesized by methods described in the following official gazettes. The synthetic methods of the compounds represented by Formula (I-2) are described respectively in Japanese Patent Application Laid-Open No. 106454/1979. The synthetic methods of the compounds of (II-2-3), (II-4-7), (II-5-5), (II-6-3) and (II-7-3) each corresponding to the examples of the compounds represented by Formulas (II-2), (II-4), (II-5), (II-6) and (II-7) are described in Japanese Patent Application Laid-Open No. 233626/1990. Further, the synthetic method of the compound of (II-7-1) as one example of the compound represented by Formula (II-7) is described in Japanese Patent Application Laid-Open No. 135445/1981. The synthetic method of the compound of (II-8-3) as one example of the compound represented by Formula (II-8) is described in Japanese Patent Application Laid-Open No. 204016/1998.
The synthetic method of the compound of (III-1-1) as one example of the compound represented by Formula (III-1) is described in Japanese Patent Application Laid-Open No. 70624/1984 or Japanese Patent Application Laid-Open No. 16940/1985. The synthetic method of the compound of (III-3-1) as one example of the compound represented by Formula (III-3) is described in Japanese Patent Application Laid-Open No. 165328/1982. The synthetic method of the compound of (III-4-1) as one example of the compound represented by Formula (III-4) is described in Japanese Patent Publication No. 46527/1987. As described above, the respective compounds of the components constituting the liquid crystal composition of the present invention can be synthesized by related arts.
The liquid crystal composition of the present invention can be prepared by conventional processes which are generally used, for example, a process in which various compounds are mixed and dissolved each other at a high temperature. For the purpose to induce a helical structure of the liquid crystal molecules to control the required twist angle, a chiral dopant such as cholesteryl nonanoate may be added to the liquid crystal composition of the present invention. Also, the liquid crystal composition of the present invention can be used as a liquid crystal composition of a guest-host mode by adding dichronic dyes of a merocyanine base, a styryl base, an azo base, an azomethine base, an azoxy base, a quinophthalone base, an anthraquinone base and a tetrazine base. Further, it can be used as a polymer dispersion type liquid crystal display and liquid crystal compositions of a birefringence-controlling mode and a dynamic scattering mode. Or, it can also be used as a liquid crystal composition of an inxe2x80xa2planexe2x80xa2swtching mode.